Microelectronic semiconductor components with integrated circuits nowadays form the basis for all kinds of electronic applications. These components comprise a complex arrangement of electronic structures which are connected up to one another in a plurality of planes arranged one above the other on a carrier substrate referred to as a chip. The joint production of chips on a substrate slice, also referred to as a wafer, is distinguished by a complicated sequence of a multiplicity of different production steps.
One important production step is so-called implantation doping or doping of a wafer. This is understood to be the targeted introduction of impurities or impurity atoms into a layer of a wafer in order to alter the properties thereof, for example the electrical conductivity and the conductivity type, in a defined manner. Electronic properties are defined for the later semiconductor components in this way.
One of the principal objectives of the semiconductor industry is to continuously increase performance by means of ever faster circuits, this being linked with continuous miniaturization of the electronic structures. However, the production of smaller structures with electrical properties virtually remaining the same imposes stringent requirements on the precision and the reliability of the production processes used. At the same time, it is necessary to rely on exact supervisory methods in order to accurately monitor the production processes.
Methods for the depth-resolved characterization of a layer of a wafer are of great significance in this case. Methods of this type are used in particular for determining a depth profile of a dopant concentration in order subsequently to check doping processes carried out on a wafer.
One known method for determining a doping profile of a layer of a wafer is based on the measurement of spreading resistances of a surface, which is referred to as spreading resistance profiling (SRP). In this method, the wafer is firstly broken in a layer region of interest and insipiently ground at an oblique angle with respect to the layer surface in order to produce an oblique ground face penetrating through the layer. Afterward, by means of two measuring tips placed onto the ground face, spreading resistances are in each case measured between linearly arranged measurement points which are at a constant distance from one another. In this case, the spreading resistances are dependent on the material properties and thus on the respective dopant concentration. In this way, it is possible to establish a depth profile of a dopant concentration.
However, the requisite breaking and insipient grinding of the wafer mean that this method is complicated and laborious. Furthermore, the wafer is thereby destroyed, as a result of which the method is cost-intensive, on the one hand, and, on the other hand, can only be employed “offline” on a small number of test wafers, and in particular not on product wafers. The consequence of this is that the measurement results may not be representative. Furthermore, the method has a relatively poor lateral spatial resolution in the mm to μm range since the spreading resistances are measured along the obliquely extending ground face. Consequently, it is not possible to determine a doping profile in a region of an individual later component in a targeted manner.
A further method used for the depth-resolved characterization of a layer of a wafer is so-called secondary ion mass spectrometry (SIMS). This involves bombarding the surface of the wafer in a layer region of interest with a primary ion beam, thereby producing a cutout in the surface with removal of semiconductor material or particles. In this case, up to about 10% of the removed particles are ionized and are referred to as secondary ions. During the removal process, the secondary ions are fed to a mass spectrometer, where they are subjected to an analysis, as a result of which dopant concentrations, in particular, are determined in temporally resolved fashion. After the removal process has been carried out, the final depth of the cutout is determined, so that it is possible, taking account of the total measurement time, to assign layer depths to the dopant concentrations recorded during the removal process. A depth profile of a dopant concentration is established in this way.
Compared with the above-described method based on the measurement of spreading resistances, secondary ion mass spectrometry has the advantage that the wafer examined is not destroyed completely, but rather only locally in a limited region, and can consequently be processed further after the removal process. Therefore, secondary ion mass spectrometry is also suitable for the “inline” measurement of product wafers.
What is problematic, however, is that the speed of material removal is dependent on the respective dopant concentration. Consequently, it is possible that the assignment of layer depths to the dopant concentrations respectively recorded, said assignment being based on an essentially constant removal speed, is inaccurate and, consequently, the established depth profile of the dopant concentration is not exact. Therefore, a doping profile obtained in this way may be only poorly suited to assessing doping processes carried out on a wafer. Furthermore, with the aid of secondary ion mass spectrometry, too, it is possible to obtain only a relatively low lateral spatial resolution in the μm range.
The so-called eddy current measuring method can be used for the completely nondestructive determination of a doping profile of a layer of a wafer. In this case, a varying magnetic field is generated by means of a coil and induces eddy currents in the surface of the wafer to be examined. For their part, the eddy currents generate inherent magnetic fields which interact with the (primary) magnetic field of the coil. By measuring a resultant change in resistance or inductance of the coil, it is possible to obtain information about material properties and thus about the dopant concentration.
What is disadvantageous, however, is that the eddy current measuring method has only a very inaccurate lateral spatial resolution and, therefore, may have to be correlated with one of the methods described above. A further disadvantage is that the induced eddy currents flow only in a relatively small depth of the wafer, as a result of which, the method furthermore has a relatively small information depth.